This invention relates generally to sputter targets used in processes for the coating of substrates, such as architectural plate glass, and to methods for manufacturing these targets. The invention is particularly concerned with (1) methods for making rotatable sputter targets comprising a sputtering material, such as graphite, that is difficult to apply to a support by plasma spray or other conventional methods and (2) to the resulting sputter targets.
Sputtering is a vacuum coating process used to deposit thin films of various materials onto the surface of a desired substrate. For example, sputtering can be used to deposit thin layers of aluminum on silicon wafers used in producing integrated circuits or to deposit metals on plastic objects used in automobiles to give the objects a metallic appearance. A common use of sputtering is to coat large plates of glass with metals so the glass plates have solar properties that enable them to be used as windows in commercial buildings.
During the sputtering process the coating or sputtering material is transported from a target or source containing that material to the substrate to be coated by bombarding the surface of the target with ions of an inert gas accelerated by a high voltage. As the gas ions hit the outside surface of the target, a portion of their kinetic energy is converted to heat while another portion is imparted by momentum transfer to the atoms of the sputtering material. These atoms gain sufficient energy to overcome their bonding energy and escape from the target surface. Upon ejection from the target surface, the atoms traverse the sputtering chamber and deposit on the substrate to form a thin film.
The sputtering process is carried out in an enclosed vacuum chamber filled with an inert sputtering gas and the target, which normally comprises the sputtering material deposited on or attached to a support. The negative terminal of a power supply, usually DC or RF power, is typically connected to the sputter target, which serves as a cathode, while the positive terminal is connected to the walls of the chamber. When the system is powered up, a negative surface charge is created on the surface of the target, thereby ejecting electrons from the sputtering material. These electrons collide with atoms of the sputtering gas stripping away electrons and creating positively charged ions. The resulting combination of positively charged ions, electrons and neutral atoms is known as the sputtering gas plasma. The positively charged ions are accelerated toward the sputter target by the electrical potential between the sputtering gas plasma and the target and bombard the surface of the sputtering material carried by the target. As the ions bombard the sputtering material, atoms of the sputtering material are ejected from the target and coat the desired substrate as it is passed through the chamber.
Frequently, this sputtering process is enhanced by placing magnets behind or near the sputtering material to influence the path taken by electrons within the sputtering chamber, thereby increasing the frequency of collisions with sputtering gas atoms. Additional collisions create additional ions and further sustain the sputtering gas plasma. The apparatus employing this enhanced form of sputtering by using strategically placed magnets is known as a magnetron system.
A magnetron sputtering system can employ as its cathode either a planar, stationary sputter target or an elongated cylindrical target that is rotatable around its longitudinal axis. A disadvantage of a planar cathode is that its surface tends to erode in a relatively narrow ring-shaped region corresponding to the shape of the closed loop magnetic field formed by the magnets. This results in frequent replacement of the target. Also, it is difficult to carry out a continuous sputtering operation onto an elongated substrate, such as plate glass. The use of rotating, cylindrical sputter targets comprised of the sputtering material itself or a support tube on which the sputtering material has been deposited as the cathode overcomes these problems and results in more effective utilization of the sputtering material. Such targets are rotated relative to the magnets to selectively bring different portions of the sputtering material on the outer surface of the target into sputtering position opposite the magnets in the sputtering chamber. Normally, a cooling fluid is passed through the center of the target to cool it as it is rotated using a rotational drive mechanism. The operation of rotatable sputtering targets (magnetrons) and their use in the sputtering process to coat substrates are described in more detail in U.S. Pat. Nos. 4,422,916; 5,096,562; and 5,922,176, the disclosures of which patents are incorporated herein by reference in their entireties.
Methods used in the past to manufacture targets comprised of a support or backing tube on which the sputtering material has been deposited include plasma spraying, casting, flame spraying and electroplating. In plasma spraying a plasma jet generated in an atmosphere of an inert gas or ambient air is used to spray coat the backing tube, whereas in flame spraying a high pressure gas is used to spray coat the backing tube with a low melting metal such as zinc or tin. Casting typically involves forming a tubular mold around the outer surface of the backing tube and introducing a molten metal in the space between the backing mold and the tube. Once the metal has cooled the mold is removed to form the target. Finally, in electroplating, the backing tube is placed in an electroplating bath and metal ions are deposited on the outer surface of the tube to form the target.
The above-described manufacturing methods are typically used to fabricate targets when a metal is to be used as the sputtering material, and some of these methods can only be used with low melting point metals. However, many instances occur where the desired sputtering material is not a metal and/or cannot be applied by these xe2x80x9cbuildupxe2x80x9d methods. For example, graphite and amorphous carbon are not metals and cannot be easily applied to the surface of a backing or structural tube. Thus, there exists a need for other methods of fabricating sputter targets, especially those that comprise non-metallic sputtering materials.
In accordance with the invention, it has now been found that rotatable sputter targets comprising graphite, amorphous carbon or other sputtering material difficult to apply to a support tube can be fabricated by (1) placing a support tube inside a prefabricated sleeve of the desired sputtering material so as to form an annular space between the outside surface of the tube and the inside surface of the sleeve, (2) filling the annular space with a thermally conductive material that flows at the fabrication temperature, which is normally the ambient temperature and usually ranges between about 5xc2x0 C. and about 45xc2x0 C., and (3) sealing the annular space at or near the ends of the sleeve to inhibit the flowable material from escaping the annular space. Usually, the material filling the annular space is a powder or other particulate material that remains in particulate form at the relatively high temperatures existing during the sputtering process and is both thermally and electrically conductive. However, the fill material may be any thermally conductive substance that can flow into the annular space at the fabrication temperature regardless of whether the material remains in the same form after the target is made. Examples of such materials include liquids and thixotropic substances such as filled epoxies that cure into a solid adhesive after the target has been fabricated.
Although the sleeve of sputtering material is normally made of graphite or other non-metallic material that is difficult to deposit on a support tube, it can also be made of any metal that is desired as a substrate coating. The support tube may be made of any material that is structurally sufficient to support the sleeve. Examples include stainless steels and carbonxe2x80x94carbon composites. Usually, the support tube is composed of a non-magnetic material.
In one embodiment of the invention, the support tube is a non-magnetic stainless steel, the sleeve is graphite, and the fill material is a graphite powder or other conductive particulate substance that is introduced into the annular space between the tube and the sleeve as the tube and sleeve are vibrated. The ends of the sleeve are typically sealed against the tube with compression rings that attach the sleeve at or near its ends to the tube and prevent the particulate fill material from leaking out of the annular space.
When the length of the sputter target is longer than that of the available sleeve of sputtering material, the sputter target can be made with a plurality of sleeves that are joined together around the support tube so that the end of one sleeve abuts the end of another. The ends of the two outer sleeves are attached to the support tube with compression rings, other mechanical devices, such as flanges and threads, or adhesives.